As shown in FIG. 7, a robot control device 1 outputs a welding voltage command Vc and a welding current command Ic to a welding machine 2. Based on these commands Vc and Ic, a controller 3 of the welding machine 2 operates the welding machine 2 to perform a welding operation.
The feed speed of a wire feeder 4 is determined by a welding current Ir, which is the output current of the welding machine 2. Therefore, control of the feed speed of the wire feeder 4 is performed based on the current command Ic supplied to the welding machine. In arc welding robots, welding conditions for the welding machine are controlled by means of an analog voltage. Therefore, in practice, the current command Ic is given to the controller 3 of the welding machine 2 in the form of a voltage (Vic) corresponding to the command Ic.
Generally, there is no linear relationship between the wire feed speed and actual welding current. In other words, the current command Ic (Vic) and the actual welding current IF have a relationship as indicated by the curve in FIG. 5, and not a linear relationship.
Conventionally, therefore, a current command Ic-actual current IF curve, which represents the relationship as shown in FIG. 5, is divided into a plurality of regions, then the curve segment in each subdivided region is approximated by a linear formula, and the actual welding current Ir (i.e., the feed speed of the wire feeder) for the command welding current Ic is obtained by using the approximate formula. That is, a linear conversion formula for deriving the command current value Ic from a target welding current is prepared for each of the welding current-based regions.
More specifically, a database is constructed beforehand such that it stores welding voltage and current data as welding conditions to be used in a welding environment including the material, diameter, etc., of a welding wire to be used, as well as a linear conversion formula uniquely assigned to the welding current-based region for calculating a current command value to be instructed to the welding machine to achieve a target welding current. The linear conversion formula is set and stored in such a manner that it is finely adjusted in accordance with the shape of a workpiece to be welded, the aiming angle of a torch, etc. During welding, the command welding voltage and the command welding current (wire feed speed command) are supplied to the welding machine in accordance with the data stored in the database.
In a welding operation utilizing such a database as described above, nearly satisfactory welding control is achieved as long as the welding conditions do not greatly deviate from the initial set values. However, in cases where the welding conditions change greatly, for example, where a different workpiece is to be welded, or the welding speed, i.e., the robot moving speed, is changed, the linear conversion formula used till then to convert the target welding current to a command current value is no longer applicable to the new welding conditions. Accordingly, the operator must carry out trial welding a plurality of times according to the new welding conditions, to obtain a new conversion formula. This work causes an inconvenience to the operator and consumes labor.